In technical standard specifications of a mobile communication system specified by 3rd Generation Partnership Project (3GPP), a method is specified to output an announcement sound based on the announcement sound transmitted from a network side after a call request is sent, more specifically, INVITE according to Session Initiation Protocol (SIP) is sent from a call-originating terminal to a call-terminating terminal and before “180 Ringing” is sent from the call-terminating terminal to the call-originating terminal (for example, Non-patent document 1). Examples of such announcement include announcing that the call is forwarded to a voice mail service from the call-terminating terminal, announcing that the call-terminating terminal is roaming abroad, and the like.
Moreover, in the 3GPP technical standard specifications, SRVCC (single Radio Voice Call Continuity) is specified, which allows a mobile station executing voice communication via Long Term Evolution (LTE) with no circuit-switched domain to make a transition to a 3G (W-CDMA) circuit-switched domain while continuing the voice communication (e.g., Non-patent document 2). As a type of the SRVCC, aSRVCC (SRVCC in alerting phase) is also specified, which enables a transition from LTE to 3G during alerting of the call-terminating terminal. The aSRVCC realizes the transition from LTE to 3G while maintaining the alerting even during the alerting of the call-terminating terminal.